There is a requirement in analog and radio frequency (RF) integrated circuits for devices known as mixers or multipliers which can take the product of two signals, not just the sum or difference as in simple analog amplifiers or analog computers. This enables the construction of variable gain amplifiers, modulators, heterodyne receivers, frequency multipliers, frequency dividers, synthesizers, and a wide variety of other signal processing functions.
Integrated circuits, such as mixers and multipliers, require the use of a nonlinear solid state device. The simplest nonlinear device employed in a signal mixer or multiplier is the diode such as used in TV satellite receivers. The nonlinear characteristics of the diode are used to obtain an intermediate frequency from the product of a local oscillator frequency and an input signal frequency. This product yields, besides the original signals, signals at the sum and differences of the local oscillator frequency and an input signal frequency. The intermediate signal frequency at the difference of the local oscillator frequency and an input signal frequency is used for down conversion of the input signal frequency to lower frequencies where it can then be more easily amplified and demodulated to remove the useful information.
Transistors are also used to fabricate integrated circuits which have the mixer and multiplier capability. Often metal-semiconductor field effect transistors (MESFET's) are employed as the nonlinear solid state device. The MESFET is typically referred to as a dual gate FET, as used in RF GaAs integrated circuits. The device structure can be understood in simple terms by considering it to be two FETs in series where dual gates of the MESFET are adjacent to each other and in series between the source and drain. Further, the drain of the first or lower transistor is in contact, internally, with the source of the upper or top device and there is no external contact to this point. The action of this device can be understood by realizing the gate to source voltage of the second or top device depends on the biasing of the gate to source voltage of the lower or bottom device. This results in the operation depending on the product of the signals on the two gates. Such GaAs dual gate FETs are typically used in a wide variety of signal processing functions at high frequencies, e.g., in the gigahertz (GHz) range including, most recently, cellular or wireless telephones.
Wireless or cellular telephones provide a good example of the shortcomings with using MESFETs in digital technology applications. That is, such digital devices require the integration of both RF and digital integrated circuit functions. Integrating analog and digital circuitry requires significant circuitry real estate and involves non-analogous fabrication steps. The push in integrated circuit technology is to develop more and more compact devices through simplified processing routines. It would be ideal to incorporate both analog and digital functions on a single chip while at the same time maintaining streamlined fabrication processes. Thus, there is a need for RF and digital integrated circuits which can be implemented on a single integrated circuit chip, e.g., a single complementary metal oxide semiconductor (CMOS) integrated circuit chip, using analogous fabrication techniques.